1. Field of the Invention
The invention relates to a motor drive circuit, and more particularly to a motor drive circuit and method with frequency setting and correcting functions.
2. Description of the Related Art
Products such as notebook computers, scanners, power supplies, and precise electronic instruments are widely used as part of a highly developed information technology industry. In general, it is very important when working with electrical products to quickly dissipate heat in order to obtain maximum efficiency. This is because when heat is concentrated inside the products and cannot be effectively dissipated, individual electrical elements, or even the entire electrical system may experience a failure. A brushless DC motor fan is usually used as a device for dissipating heat so that the electrical elements within the system may operate normally under the optimum temperature environment.
Please refer to FIGS. 1A to 1B, which respectively show a conventional drive circuit for a brushless DC motor and a flow chart for a method of driving the motor. In step 106, a Hall sensor is first used to sense the magnetic field distribution of a rotor magnet 104 and to generate a sense signal. Then, in step 108, a controller 100 is used to receive the sense signal and then generate a non-corrected control signal. In step 110, an actuator 102 amplifies the control signal power and switches the current direction to change the polarity of the coil-induced magnetic field. Finally, in step 112, the magnetic force generated by the coil-induced magnetic field with respect to the rotor magnet 104 is used to drive the rotor magnet 104 to rotate in a specific direction.
Consequently, the conventional method for driving the brushless DC motor is of an open-loop type. That is, the controller can only output a non-corrected control signal in a one-way manner. The non-corrected control signal is then transferred to the actuator 102 in order to drive the motor to rotate. Hence, the motor speed is determined by the driving capability of motor driver and the loading of motor. Driver can not control the motor speed with the conventional driving method. It only depends on the motor mechanical parameter.
Next, when the conventional brushless DC motor is operating, although the motor rotation speed is constant within the primal time period, the motor may lose its optimum standard rotation frequency after a period of time has elapsed. In this case, the motor rotation speed may wander, resulting in undesired phenomena (e.g., unstable gas quantity, loud motor noise, and vibration), and the system operation may be negatively influenced.
The conventional brushless DC motor may have inherent structural defects from the manufacturing processes, such as mechanical assembly errors, coil shape variations, washer deformations, and poor bearing lubrication. Therefore, a single batch of motors may have individual units with different standard rotation speeds, thus manufacturing costs will increase if a high percentage of the products cannot meet the standard requirements and have to be eliminated.
Brushless DC motors in computer systems operate at a precise standard rotation frequency by using the computer to perform rotation frequency detection and then correcting the rotation frequency according to the detected rotation frequency. In other words, the brushless DC motor itself does not have the capability of direct detection and correction. Furthermore, the loading of the computer system is inevitably increased if the computer system has to frequently detect the motor rotation frequency in order to maintain the ideal motor rotation speed.
Consequently, the standard rotation frequency of a conventional brushless DC motor may wander and vary during the operation causing the system to operate abnormally. Moreover, structural errors due to the manufacturing processes may even deviate the real rotation speed from the standard rotation speed.